Method of producing an aluminium surface with a high total reflectance

ABSTRACT

A method of producing a reflector sheet, which method comprises treating an Al alloy sheet to increase the total reflectance of a surface of the sheet for use as a lighting reflector by bringing the sheet into contact with an acid or alkaline fluid that dissolves aluminium metal, said fluid having a viscosity of less than 0.01 Pa·s, under conditions to remove from 10 nm to 2000 nm of metal from the surface, and cutting or forming the treated Al alloy sheet into the shape of a reflector sheet.

This invention is concerned with a method of treating an aluminium alloysheet having a surface to increase the total reflectance of thatsurface. Total reflectance is an important property for lightingreflector sheet. Total reflectance is the proportion of light that isreflected from rather than absorbed by the sheet surface. Themanufacturers of lighting products want high total reflectance becausethat represents good energy efficiency. On the other hand, high or lowspecularity products may be preferred depending on function. For ahighly specular surface, a high proportion of the incident light isreflected at the same angle as the angle of incidence. Low specularityor highly diffuse reflectance involves light reflected over a wide rangeof angles. Often rather matt surfaces are favoured and terms such as“reflectormatt” and “semi-specular” are used to describe currentproducts with a perceived optimum level of diffuse reflectance.

After silver, pure aluminium has an outstandingly high intrinsic totalreflectance property. The total reflectance of aluminium alloys isgenerally lower than of the pure metal by an amount related to theconcentration of the alloying ingredients. Pure aluminium metal anddilute aluminium alloys (typically 99.8% Al or purer, for exampleAA1080) can be bright rolled to give surfaces having excellent totalreflectance. But pure aluminium has poor mechanical properties, andbright rolling is relatively expensive and necessarily results in sheethaving a highly specular surface. There is a need for production routesto achieve surfaces having correspondingly high total reflectance, butusing more concentrated aluminium alloys having better mechanicalproperties, and using rolling conditions that are less critical.

These needs have been addressed by production techniques that involvechemical etching and/or electrochemical polishing. Chemical andelectrochemical polishing and brightening techniques involve the use ofviscous solutions based on concentrated phosphoric acid, and they resultin removal of metal from the surface, to a depth of more than 1 μm andtypically of around 10 μm. The high viscosity appears necessary in orderto achieve a static fluid film adjacent the surface by means of whichthe surface is flattened. Such techniques are described in WO 99/13133and WO 99/13134. A viscous chemical brightening solution based onconcentrated phosphoric acid is marketed under the trade mark Phosbrite159. But the use of such viscous solutions is undesirable, for they aredifficult to remove rapidly from a surface in a manner consistent withhigh-speed treatment of that surface. The removal of many microns ofmetal from the surface is also undesirable, for it is expensive in powerand chemicals, it creates effluent problems, it thins and weakens thesheet being treated, and has other disadvantages as discussed below.

WO-A-9913133 describes the removal or reduction of directionality oranisotropy from aluminium surfaces which involves a two-step process ofchemical etching and electrochemical polishing. A total process time ofover 60 seconds is disclosed. In addition, the solutions used in theprocess have a high viscosity and concentration which would causeproblems with drag-out. The process described would remove a significantamount of metal from the surface.

A thesis by Inger Lindseth of the Norwegian University of Science andTechnology (published 16 Nov. 1999) entitled “Optical Total Reflectance,Near-Surface Microstructure, and Topography of Rolled AluminiumMaterials” discloses etching of aluminium alloys and investigates thetotal reflectance of industrially cold rolled and etched materials. 85%phosphoric acid solution is used as the etching solution, which has arelatively high viscosity.

Symposium II—Surface and Near-Surface Analysis of Materials, June 1998,pages 49 ff (9^(th) Cimtec—World Forum on New Materials) relates to aninvestigation on how the surface topography and microstructure ofdifferent aluminium surfaces affect the total reflectance of light. TheAA1070 alloy is cold rolled commercially and then mechanically polishedand electropolished. The paper teaches the improvement of TotalReflectance by removing the highly deformed surface layer. In addition,a batch process is disclosed.

A number of documents have investigated effects on SpecularReflectivity, in contrast to Total Reflectivity. These include U.S. Pat.No. 4,247,378 and GB-A-740880. There is a distinction between totalreflectance, specular reflectance and diffuse reflectance. Totalreflectance is used to describe the total amount of light reflected froma surface, as opposed to that light which is absorbed by the surfaceupon which it falls and slightly warms the surface by being absorbed. Itis expressed as a percentage of the incident light intensity. To measuretotal reflectance it is therefore necessary to try and capture thereflected light at every possible angle of reflection. In practice, thisis typically done by using an integrating sphere. The sphere has a smallaperture in its base that is placed against the surface to be measuredand two other apertures; one for introducing a beam of light and one formeasuring the integrated sum of the reflected light. The surfaces ofsuch spheres are made of a highly reflecting substance such as bariumsulphate to ensure that they do not contribute to the measuredabsorption. After the beam of light has struck the surface to bemeasured it is reflected within the sphere and, regardless of the angleit was reflected at, eventually detected and absorbed by the measuringdevice. By comparing the intensity of light measured at the detectorwhen the sample aperture is plugged with the same material as the innerwall of the sphere and when the beam is reflected off the sample surfacethe total reflectivity, or percentage absorption, can be determined.

Specular reflectance is used to describe the mirror-like properties of asurface. For a completely specular surface all the light that isreflected, and not absorbed, is reflected at an angle of reflection thatis the same as the angle of incidence. In other words, there is noscattering of the light to different angles of reflection. Specularsurfaces may have high total reflectivity, but there is no directcorrelation between these two parameters. For example, a highly specularsurface may absorb quite a lot of the incident light. To measurespecularity it is necessary to limit the light measured at the detectorto that which is at or very close to the true specular angle. Adeviation from the specular angle of 2 degrees from the true specularangle is sometimes used for this measurement.

Diffuse reflectance describes light that is not reflected at thespecular angle and such surfaces are said to be matt. These surfacesscatter the incident light to many different angles of reflection. Asimple definition of diffuse reflectance is that it is the totalreflectance less the specular reflectance. To quantify the true diffusenature of a flat surface it would be necessary to measure the reflectedlight at all the possible angles of reflection. In practice onlyselected angles are generally used.

GB-A-718024 relates to a method of chemically treating aluminiumsurfaces for the purpose of increasing their specularity.

U.S. Pat. No. 2,847,286 relates to a method of forming a glossy(specular) surface on an aluminium body which includes treatment with anaqueous solution containing as active ingredients nitric acid,hydrofluoric acid and lead ions. The use of lead ions is essential tothe process disclosed. A batch process is used.

JP-A-05112900 discloses the etching of the surface of an aluminium sheetwith the use of an electrolyte which is an aqueous neutral saltsolution. The invention disclosed relates to a lithographic sheet.

It is an object of the present invention to provide a method ofimproving the total reflectance of a surface of an aluminium alloy whichdoes not involve treatment with a high viscosity fluid.

The invention provides a method of producing a reflector sheet, whichmethod comprises treating a continuous Al alloy sheet to increase thetotal reflectance of a surface of the sheet to a value of at least 85%by bringing the sheet at a speed of at least 50 m/min into contact withan acid or alkaline fluid that dissolves aluminium metal, said fluidhaving a viscosity of less than 0.01 Pa·s, wherein from 10 nm to 2000 nmof metal is removed from the surface, and cutting or forming the treatedAl alloy sheet into the shape of a reflector sheet.

The invention also provides a reflector sheet formed by the method,wherein the surface has a total reflectance of at least 85%.

According to a further aspect of the present invention, there isprovided the use of a treated Al alloy sheet as a reflector sheet,wherein the treated sheet is formed from a continuous Al alloy sheet bybringing the sheet at a speed of at least 50 m/min into contact with anacid or alkaline fluid that dissolves aluminium metal, said fluid havinga viscosity of less than 0.01 Pa·s, wherein from 10 nm to 2000 nm ofmetal is removed from the surface and the total reflectance of a surfaceof the sheet is increased to at least 85%.

According to a further aspect of the present invention, there isprovided a method of producing a reflector sheet, which method comprisestreating an Al alloy sheet to increase the total reflectance of asurface of the sheet to a value of at least 85% by treating the sheetwith an acid or alkaline fluid that dissolves aluminium metal, whereinfrom 10 nm to 2000 nm of metal is removed from the surface, and cuttingor forming the treated Al alloy sheet into the shape of a reflectorsheet.

Preferably, no more than 1500 nm is removed. More preferably no morethan 1000 nm is removed, and even more preferably no more than 500 nm isremoved. A preferred range is 20 nm to 500 nm. The smaller amounts arepreferred for processing reasons.

For environmental reasons, heavy metals (and in particular, lead) areexcluded from the treatment process. In addition hydrofluoric acid, ifit is present, should preferably be present in amounts not greater than500 ppm.

The treatment involves the use of a non-viscous acid or alkaline fluid,with or without the imposition of an applied potential, which dissolvesthe surface. It might have been anticipated that such solutions wouldetch the surface and make it rougher which would increase diffusereflectance and, because of multiple surface reflections, reduce totalreflectance. However, the inventors have found that low levels of metaldissolution can lead to a significant increase in total reflectance.There follows a tentative and partial explanation of this surprisingtechnical effect.

For metals having comparable surface cleanliness and comparable surfaceroughness, total reflectance (TR) is largely determined by chemicalcomposition. As previously noted, pure aluminium has a very high TR.However the TR is reduced for untreated alloys due to the presence ofsecond phase constituents and solid solution elements at the surface.Absorption/reflection probably takes place within a surface layer toabout 20 nm thick. A perfectly flat surface absorbs light in an idealmanner depending on chemical composition. However light may undergomultiple absorption/reflectance events before it can escape from roughsurfaces, thus reducing the TR.

Additionally, rolled surfaces have disturbed surface microstructureswhich may include fine grains with grain boundary segregated species,sub surface residual lubricant, and sub surface oxides. It is expectedthat all these may perturb the optical properties of the surface. Thusthe removal of these disturbed layers would be expected to increase theTR. However, it is also known that as an aluminium alloy surface isdissolved, solid solution elements which are less reactive thanaluminium accumulate in a thin layer at the surface. The amount ofaccumulation depends on the composition of the alloy and the extent ofdissolution. But at the dissolution levels contemplated in the priorart, this accumulation of surface contaminant can have a major andadverse effect on TR.

Furthermore, dissolution of aluminium alloy surfaces is not generallyuniform. This depends on microstructural and composition features.Grains of different orientation may dissolve at different rates. Grainboundaries or fortuitous scratches may be active sites for preferentialdissolution. Second phase particles can set up microgalvanic corrosioncells. These non-uniformities lead to varying degrees of surfaceroughness. Thus, excessive metal dissolution is liable to increase thesurface roughness which may limit the TR achievable.

The main use of the products of this invention will be for reflectorsheet used where high reflectance of electromagnetic radiation isrequired i.e. visible light or light close to the visible range forexample infra red light. Such products include lighting reflectors, andparticularly the construction of louvres for indoor ceiling units.Similar applications may include solar receptors where efficient captureof sunlight by an absorber to which it is directed by reflectingsurfaces is important. The product may have application for decorativeeffects on packaging products such as beverage cans. Also, there areexpected to be other applications where low energy absorption byaluminium surfaces is critical, e.g. laser processes and hightemperature applications.

The Al alloy is preferably of the AA1000 or AA3000 or AA5000 or AA6000series (of the Aluminium Association Register), and certain of the M8000series alloys may be appropriate, such as for example AA8006. Themaximum purity of the starting aluminium is preferably 99.85%. However,an advantage of the invention is that it is possible to use a less pure,less expensive starting aluminium having better mechanical propertiesthan was previously possible, for example AA1200 (Al greater than 99.2%)or AA1050 (Al greater than 99.5%) or even a recycled alloy of typeAA3105. The Al alloy sheet is generally a rolled sheet. Preferably, thesheet is cold rolled, and possibly annealed, before any treatment withthe solution. As well known in the art, rolling conditions may be chosento produce a bright finish or a semi-bright finish. According to theinvention, even mill finish is useable. One useful technique is packrolling, in which (for example) two sheets are passed together throughthe nip of the roller, resulting in two rolled sheets each having asmooth surface (that came in contact with a surface of the roller) and apack rolled surface. It is of course possible to pack roll more than twosheets simultaneously, in which case all internal sheets will have twopack rolled surfaces. As discussed below in more detail, it hassurprisingly been found that pack rolled surfaces, which are generallymatt, nevertheless have a higher TR than the bright surfaces.

The surface, generally a rolled surface, may need to be cleaned toremove surface contaminants particularly rolling lubricant. Then thesurface is subjected to the action of an acid or alkaline fluid having aviscosity of less than 0.01 Pa·s. preferably 0.005 Pa·s. This viscosityis measured at the temperature of use, generally an elevatedtemperature. Viscosities at ambient temperature are correspondinglyhigher, e.g. up to 0.4 Pa·s. preferably up to 0.2 Pa·s. A low viscosityat the temperature of use permits the treating solution to be quicklyand easily removed from the treated surface.

It is preferred that the total concentration of dissolved species in thetreating solution is below 40% by weight.

The Al sheet is brought into contact with the acid or alkaline fluid ata speed of at least 50 m/min. Speeds greater than 50 m/min are suitablefor the low viscosity acid or alkaline fluid with which this inventionis concerned; but they are not suitable with viscous fluids such asthose conventionally used for brightening or polishing. Treatment speedsof greater than 50 m/min up to 600 m/min or even more may be achieved inexisting equipment for the continuous treatment of rolled sheet. Contactwith the acid or alkaline fluid may be effected by passing the sheetthrough a spray of the fluid or more usually by passing the sheetthrough a bath containing the fluid, said bath being of a length chosenin relation to the speed of movement of the sheet to provide a suitablecontact time, preferably less than 30 seconds, for example less than 20seconds, preferably less than 10 seconds, for example 0.1–5 seconds. Ina preferred embodiment, the method of the invention is carried out as asingle step treatment.

A wide range of acid or alkaline solutions can be used. Sulphuric acidis effective, and small additions of hydrofluoric acid may be made tofacilitate dissolution—this is the basis of the Ridolene systems used inthe examples. Phosphoric acid is effective when used under conditions toclean rather than to anodise the surface. Caustic soda is effective andas in the examples below may be used in conjunction with sodium nitrate.Preferably the aluminium concentration is kept low enough to prevent anundue increase in the viscosity, for example below about 150 g/l,preferably below 120 g/l. These solutions are collectively known asetching cleaners or etchers, terms which imply roughening in contrast tothe smoothing effect of brightening or polishing solutions. Thesesolutions are capable of roughening Al alloy surfaces and so reducingthe TR; but are used under conditions to effect a low level ofdissolution required to increase TR; and at a rate compatible with theeconomic operating speeds of available equipment. Clearly increasingsolution concentration or temperature will increase the dissolutionrate. Similarly electrolytic treatments will work faster than simplechemical dissolution. A feature of the present invention is thatconditions are chosen to remove so little metal that any matting effectdoes not reduce the TR of the surface.

Specifically, the method is performed to remove from 10 nm to 2000 nm,preferably 20 nm to 500 nm, of metal from the surface. Metal removal mayconveniently be measured by gravimetric means, which gives an averageover the whole surface, using the density of the aluminium alloy. Thisignores any influence that entrained oil or oxide or other impuritiesmay have. Account may need to be taken of any non-metallic material,e.g. oxide or hydroxide or oil on the surface. The starting weight, inthe gravimetric determination, is obtained on a fully degreased surfaceso that undue influence from the presence of grease or rolling oils iseliminated. For example the surface may be cleaned with acetone oranother suitable solvent or cleaner that does not dissolve aluminium.Surface oxide may, where necessary, be removed by cleaning in a solutionof chromic/phosphoric acid before the gravimetric determination is made.Metal removal may be by chemical dissolution, or may beelectrochemically assisted e.g. by applying an AC or DC potential withthe Al alloy product as the anode. The AC potential may be of anyselected wave form known in the art, for example sinusoidal or pulsed inany convenient manner. The AC potential may be biased in either apositive or a negative direction. Because very little metal is removed,a contact time between the fluid and the surface may be rather short,for example less than 20 seconds and even under some circumstances lessthan 1 second. Such contact times lend themselves readily to continuoustreatment. After a desired contact time has elapsed, the surface may bewashed, and it is an advantage of the invention that the low viscosityof the acid or alkaline fluid makes its removal quick and easy.

The concentration and the temperature of the acid or alkaline fluids, aswell as the extent of agitation and other reaction conditions, may beadjusted together with contact time to achieve a desired degree of metalremoval. The theoretical maximum TR for an aluminium metal surface isabout 91 to 92%. The lighting industry generally requires that surfacesto be used for lighting reflectors have a TR of at least 80% preferablyat least 85%. Aluminium surfaces are soft and easily scratched, and somay require a coating for protection from superficial damage duringservice. Alternatively, the surface can be left unprotected in certainapplications, for example indoor lighting reflectors. There is a largevolume of literature on organic and inorganic protective coatings onaluminium alloy surfaces to be used for lighting reflectors. An anodicaluminium oxide coating can readily be formed on the surface, but thisreduces the TR by about 5–6%. Organic or inorganic lacquers can beapplied to protect the metal surface, but these also reduce the TR. Itis therefore desired that a method of treating an aluminium alloysurface should increase its TR to at least about 90%. Given a suitablesubstrate surface, existing commercial methods e.g. based on Phosbrite159 are capable of doing that. As shown in the examples below, themethod of the present invention is also capable of doing that, butwithout many of the disadvantages to which the existing commercialtechnology is subject.

Rolled sheet including pack rolled sheet generally has surface markingsextending longitudinally or transversely to the rolling direction. Ithas been thought that etching after-treatments should be sufficient toobliterate those directional markings, so that the treated sheet hasisotropic optical properties. That is part of the reason why existingetch treatments are designed to remove so much surface metal. Thepresent inventors have determined that a degree of surface opticalanisotropy need not be detrimental, from a technical or an aestheticviewpoint. Thereby they have been enabled to remove only small amountsof metal from the Al surface, without necessarily obliteratingdirectional markings that resulted from rolling, but also withoutencountering the problems to which conventional smoothing etchtreatments are subject.

The specular gloss and the roughness are properties of a surface thatare to some extent associated. The specular gloss of sheet used inlighting reflectors is important from an aesthetic standpoint, anddifferent manufacturers have different requirements. Thechemical/electrochemical treatments with which this invention isconcerned may decrease the specular gloss of the starting surface. Inany case, the treatments do not significantly increase the speculargloss. The chemical/electrochemical treatment usually increases theroughness of the surface. It is rather surprising that this increase inroughness can be accompanied by an increase in total reflectance.

The treated sheet may be cut or formed into the shape of a lightingreflector.

The following examples illustrate the invention. Treatment conditions inlaboratory experiments are designed to mirror those that would be usedin plant treatment of continuous coil.

Data relevant to the following examples is as follows: Phosbrite 159(73% phosphoric acid, 12% sulphuric acid, 6% nitric acid, 8% water): 50cp (0.05 Pa·s) at 20° C. Using the Bohlin viscometer it was measured as22 cp (0.022 Pa·s) at 100° C.

Ridolene: Bohlin viscometer measurement 4 cp (0.004 Pa·s) at 60° C. Bookvalues: water at 60° C.—0.734 cp (0.000734 Pa·s), water at 20° C.—1.002cp (0.001 Pa·s).

Book values: concentrated sulphuric acid at 20° C.—24.8 cp (0.0248Pa·s), at 0° C.—50 cp (0.05 Pa·s).

Ridolene 124 made up at 10 ml/l contains about 3.3 ml/l sulphuric acid.A further 9.8 ml/l sulphuric acid is added to increase the totalsulphuric acid concentration by a factor of 4.

Ridolene 120E made up at 2 ml/l gives a free fluoride concentration of25 ppm in the bath, which operates normally at 60° C.

EXAMPLE 1

Samples of bright rolled 1060 sheet were variously treated. TR, gloss,metal dissolution were measured. The conditions and results are given inTable 1 below. Phosbrite 159, Minco, and Ridolene are proprietaryformulations for chemical brightening, degreasing and cleaningrespectively. The metal dissolution is expressed as a distance into thesurface, which was determined from gravimetric data, and is thus anaverage over the whole surface.

TABLE 1 METAL TREATMENT TIME % TR DISSOLUTION (nm) None 79.2 21Mechanical polish + 71.1 acetone degrease CrO₃/H₃PO₄ 30 min 86.4 Mincodegrease 10 min 82.3 Phosbrite 159 2 min 90.1 10 ml/l Ridolene 124, 10 s88.5 37 2 ml/l Ridolene 120E, 20 s 90.9 56 9.8 ml/l H₂SO₄, 60° C. 30 s88.0 102 20% H₃PO₄, 1 s 86.5 200 3000 A/m²ac, 90° C. 3 s 90.5 300 5 s88.0 350 5% NaOH/4% 5 s 86.4 35 NaNO₃/100 g/l Al 10 s 87.3 70

Results for the bright rolled surface are given for comparison.Mechanical polishing alone appears to degrade the surface compared tothe bright rolled condition.

There are also a series of other comparisons. The CrO₃/H₃PO₄ processdissolves any surface oxides but not the substrate metal, and thus hasnegligible smoothing capability. This process increases TR, but does notaffect specularity and roughness. This suggests that the presence ofsurface oxide affects TR. The Minco degrease is also non-etching.Results show the same trends as for the CrO₃/H₃PO₄ solution.

The Phosbrite treatment significantly increased the TR, as expected.

The Ridolene and ac H₃PO₄ process gave an increase in TR which may be asgreat as achieved using the conventional technology, Phosbrite.

The results for the alkaline etch, Ridolene and ac H₃PO₄ processesindicate that, by appropriate choice of (electro-) chemical treatment,surfaces with high TR can be achieved. It should be noted that theaverage amounts of metal removed by these processes vary considerably,which is surprising given the expectation that the success of theseprocesses depends on the removal of a disturbed layer.

EXAMPLE 2

The inventors have measured the TR of pack-rolled AA1050A sheet both onthe matt and bright surfaces, and again after a 20 s Ridolene treatment.The samples were obtained from trials where 30, 40 and 50% thicknessreductions were achieved by the pack-roll pass. The matt surface afterthe 50% reduction was substantially non-directional. The TR results arein Table 2 below. Note that there are duplicate results for a 50%reduction.

TABLE 2 % TR AFTER % REDUCTION ON SUBSEQUENT PACK ROLLING SIDE OF STRIP% TR RIDOLENE 50 Matt 86 87 50 Bright 78 83 50 Matt 81 88 50 Bright 7582 40 Matt 77 85 40 Bright 69 78 30 Matt 76 82 30 Bright 69 77

Rather surprisingly, it was found that the matt surfaces have a highertotal reflectance than the bright surfaces. Also, it appeared thathigher values were achieved by greater reductions. Further benefitsarose from the Ridolene treatments.

TR measurements were made on a series of pack rolled samples of AA1050Asheet. Here, there were reductions of 30 to 90% on the first pass,followed by interannealing at 200° to 500° C., with final pack rollingreductions of 35 or 50%. The TR results from the matt surfaces are givenin Table 3 below. The results shown in both Tables 3 and 4 were alsotaken after the cleaning process with Ridolene for 20 s.

The data shows that a TR up to 91–92% can be achieved; this is thetheoretical maximum level for aluminium. It appears that the optimumconditions to achieve a high TR are a 73% first pass reduction,interanneal no higher than 450° C., and a 50% reduction on pack rolling.

TABLE 3 FIRST PASS REDUCTION, % 30 50 73 90 PACK ROLL PACK ROLL PACKROLL PACK ROLL TEMP, REDUCTION, % REDUCTION, % REDUCTION, % REDUCTION, %° C. 35 50 35 50 35 50 35 50 200 87 90 87 90 89 91 88 88 320 79 82 89 9088 90 84 91 400 86 85 89 91 92 90 86 89 450 87 88 89 91 91 91 83 86 50078 84 74 82 89 89 82 81 550 73 77 76 74 88 83 84 83

Variations in specularity also arose. Table 4 below gives the 20° glossvalues on the pack-rolled (matt) surfaces.

TABLE 4 FIRST PASS REDUCTION, % 30 50 73 90 PACK ROLL PACK ROLL PACKROLL PACK ROLL TEMP, REDUCTION, % REDUCTION, % REDUCTION, % REDUCTION, %° C. 35 50 35 50 35 50 35 50 200 16.8 13.6 22.9 16.5 17.9 17.8 49.2 39.4320 11.9 10.3 13.7 11.6 11.5 11.1 25.8 15.9 400 12.6 11.5 14.5 11.4 13.59.5 21.4 14.9 450 12.7 10.2 16.2 12.8 12.5 9.6 21.1 12.3 500 12.0 10.610.4 9.9 11.3 8.9 17.8 10.0 550 9.6 8.5 8.5 6.8 14.9 10.2 31.5 15.4

The gloss values indicate a diffuse reflector. It appears that the bestdiffuse reflectors were given by a 30% first pass reduction, aninteranneal at about 500° C., and a 50% pack rolling reduction. Thus, tosome extent the conditions for high total reflectance and low glosscoincide.

EXAMPLE 3

Some of the brightened examples were exposed indoors for some months:the change in Total Reflectance was as follows:

Total Reflectivity (TR) on Office Exposed Sheets

The samples exposed for 8 months were:

-   1. AA1060 bright rolled commercially produced material+20 s    Ridolene: Original 89% TR, 8 months 87% TR-   2. AA1060 as rolled: Original 82% TR, 8 months 79% TR-   3. AA1050A+20 s Ridolene: Original 88% TR, 8 months 88% TR-   4. AA1050A treated electrolytically in phosphoric acid: Original 83%    TR, 8 months 84% TR-   5. AA1050A pack rolled+20 s Ridolene: Original 88% TR, 8 months 87%    TR

The results of these tests show that the high total reflectivityachieved by applying methods of this invention is sustained over anextended period of time without significant degradation in performance.

1. A method of producing a reflector sheet, which method comprisestreating a continuous Al alloy sheet to increase the total reflectanceof a surface of the sheet to a value of at least 85% by bringing thesheet at a speed of at least 50 m/min into contact for less than 30seconds with an acid or alkaline fluid that dissolves aluminium metal,said fluid having a viscosity of less than 0.01 Pa·s, wherein from 10 nmto 2000 nm of metal is removed from the surface, and cutting or formingthe treated Al alloy sheet into the shape of a reflector sheet.
 2. Themethod as claimed in claim 1, wherein from 20 nm to 500 nm of metal isremoved from the surface.
 3. The method as claimed in claim 1, whereinthe Al alloy is a AA1000 or AA3000 or AA5000 or AA6000 or AA8000 seriesalloy.
 4. The method as claimed in claim 1, wherein the sheet is packrolled sheet, a matt surface of which is subjected to the action of theacid or alkaline fluid.
 5. The method as claimed in claim 1, wherein thesurface of the sheet is subjected to electrolysis in the presence of thefluid.
 6. The method as claimed in claim 1, wherein the treated surfaceis given a protective organic or inorganic coating.
 7. The method ofclaim 1, wherein the treated Al alloy sheet is cut or formed into theshape of a lighting reflector.
 8. A method of producing a reflectorsheet, which method comprises treating an Al alloy sheet to increase thetotal reflectance of a surface of the sheet to a value of at least 85%by treating the sheet with an acid or alkaline fluid that dissolvesaluminium metal for less than 30 seconds, wherein from 10 nm to 2000 nmof metal is removed from the surface, and cutting or forming the treatedAl alloy sheet into the shape of a reflector sheet.